Twenty-seven substances, including antineoplastic and immunosuppressive agents, contaminants of human foodstuffs, rodent carcinogens, pesticides, and artificial sweeteners are being evaluated in four species of non-human primates for their potential carcinogenicity and other long-term toxic effects. Eighteen of these substances have not as yet demonstrated carcinogenic activity, although some have been on test for less than one year. Nine of the compounds are carcinogenic in non-human primates, producing tumors in 9.1-100% of the treated animals. 1-Methyl-1-nitrosourea induced squamous cell carcinomas of the oropharynx and esophagus, with the esophageal tumors possessing clinical and morphologic similarities to human esophageal carcinoma. Long-term treatment with procarbazine resulted in an increased incidence of malignancies, approximately one-half of which were AML. One of 10 monkeys treated with adriamycin also developed AML. The effects of six of the nine compounds DENA, DPNA, 1-nitrosopiperidine, aflatoxin B1, MAM-acetate and urethane were manifested primarily as hepatocarcinogenicity. Alpha-fetoprotein has been found to be a useful marker for diagnosing and for following the treatment of hepatocellular carcinoma in non-human primates.